The present invention relates to U.S. Pat. No. 7,605,805 which was applied by the same applicant.
Korean Patent Application No. 2005-23382 discloses a touch sensor as shown in FIG. 1, which senses whether or not a touch object is in contact with the touch sensor by varying a difference in delay time between a touch signal and a reference signal using the electrostatic capacitance of the touch object.
Referring to FIG. 1, the touch sensor includes a reference signal generator 10, a first signal generator 21, a second signal generator 22, a touch signal generator 30, and a low pass filter (LPF) 40. Specifically, the reference signal generator 10 generates a reference signal ref_sig. The first signal generator 21, which includes a resistor R11 and a capacitor CAP, delays the reference signal ref_sig by a constant delay time irrespective of whether or not the touch object is in contact with the touch sensor, and generates a first signal sig1. The second signal generator 22, which includes a resistor R12 and a touch pad PAD, delays the reference signal ref_sig by a variable delay time according to the electrostatic capacitance of the touch object and generates a second signal sig2. The touch signal generator 30, which includes a D-flip-flop, latches the second signal sig2 in response to the first signal sig1 and generates a touch signal con_sig. The LPF 40 filters the touch signal con_sig and outputs a filtered signal.
The touch signal generator 30 generates a touch signal con_sig having a first level when the touch object is brought into contact with the touch pad PAD and the second signal sig2 has a longer delay time than the first signal sig1. On the other hand, the touch signal generator 30 generates a touch signal con_sig having a second level when the touch object is out of contact with the touch pad PAD and the second signal sig2 has a shorter delay time than the first signal sig1.
As described above, the touch sensor of FIG. 1 varies a difference in delay time between the first signal sig1 and the second signal sig2 depending on whether or not the touch object is in contact with the touch pad PAD.
However, when the touch pad PAD has poor touch sensitivity or the touch object has very small electrostatic capacitance, a difference in delay time between the first signal sig1 and the second signal sig2 cannot be sufficiently varied, so that a malfunction may occur in the touch sensor.
Furthermore, the impedance of a circuit device included in each of the first and second signal generators 21 and 22 and a delay difference between the first and second signals sig1 and sig2 may vary with operating conditions of the touch sensor, such as an operating power supply voltage and the temperature and humidity of the atmosphere.
However, although the impedance of the circuit device included in each of the first and second signal generators 21 and 22 is changed according to the operating conditions, the conventional touch sensor provides no calibration element. As a result, the operating characteristics of the touch sensor are varied according to the operating conditions and, what is worse, a malfunction may occur in the touch sensor.